This invention relates generally to optical systems and more particularly to optical systems adapted to provide either a relatively wide field of view or relatively narrow field of view within a relatively small packaging volume, i.e., in the order of less than 100 cubic inches.
As is known in the art, optical systems are used in a wide range of applications from cameras to missile system seekers. In a missile system application, the optical system is disposed in front of a detector, such as an infrared detector, for focusing infrared energy from a target onto the detector. In some systems, the optical system is required to have a different field of view in different phases of the missile's flight. Further, because of the relatively small space provided within the missile, packaging constraints limit the achievable resolutions and fields of view attainable with many optical systems. High resolution infrared systems in combination with large field of view requirements force large apertures and focal lengths which are not consistent with small packaging constraints.
For example, in one application a four to one change in field of view is required. Referring to FIG. 1, a conventional high resolution refractive optical system 10 is shown for directing energy onto a detector 12, here a focal plane array of detector elements for processing by a processor 15. Here, the focal length of the optical system 10 shown in FIG. 1 is 200 millimeters (mm). The refractive optical system 10 shown in FIG. 1 has two sets 14a, 14b of refractive lenses with a region 13 therebetween.
Referring now to FIG. 2, the system 10 is shown shortened by moving lens set 14b closer to lens set 14a. Here, the focal length is 50 mm and provides a larger field of view optical system 10'. It is first noted that the system 10' has a lower resolution that the optical system 10 (FIG. 1 ). It is also noted that in region 13' of optical system 10' (FIG. 2) the ray angles are steeper and of larger width than in region 13 (FIG. 1) due to the four to one reduction in focal length. Further, an additional constraint unique to infrared imaging systems is the placement of the optics aperture stop within the cooling dewar volume to reduce extraneous background radiation thereby improving seeker sensitivity. Given this constraint, it becomes extremely desirable to utilize a re-imaging optics configuration to prevent very large optics aperture requirements.
Referring now to FIG. 3, a conventional high resolution catadioptric optical system 10" is shown. System 10" includes a Cassegrainian optical arrangement having a primary reflector 20 and a secondary reflector 22. The optical system 10" includes a refractive optical system 24 and the detector 12, here a focal plane array of detector elements. The primary reflector 20, secondary reflector 22 and refractive optical system 24 are arranged to direct energy to the detector 12. Thus, the system 10" is a high resolution catadioptric optical system with a compressed mechanical length of half the focal length of the optical system 10 shown in FIG. 1. Referring now to FIG. 4, the optical system 10" (FIG. 3) is shown with the secondary reflector 22 moved rearward closer to the refractive optical system 24. The resulting optical system 10"' has theoretically a focal length of 50 mm and is one fourth in length as compared to the length of system 10" (FIG. 3). However, it is noted that there is increased blockage in the rays and the local f# between the reflectors is thereby reduced resulting in a design which is not practical.
In summary, with a system required packaging length less than the focal length of the optical system, a refractive, high resolution, optical system is impractical (FIG. 1), while a catadioptric system (FIG. 3) may be used. When a focal length of one-fourth is now required, movement of the refractive optical system portion of the catadioptric system towards the secondary reflector results in an impractical design (FIG. 4) because of a large increased blockage of incoming light and a severe growth in required diameter of the primary reflector. Further, the resulting optical system (FIG. 4) will present a large change in effective f# because there is a large change in the amount of energy intercepted by (i.e., impinging on) the detector due to increased blockage.